Heat-Treated GlassBack to Glass Textbook
In order to provide greater resistance to thermal and mechanical
stresses and achieve specific break patterns for safety glazing
applications, annealed float glass products may be subjected to a
heat-treating process. The most commonly used process for
heat-treating architectural products calls for glass to be cut to
the desired size, transported through a furnace and uniformly heated
to approximately 1150° F (621° C). Upon exiting the furnace, the
glass is rapidly cooled (quenched) by blowing air uniformly onto
both surfaces simultaneously. The cooling process locks the surfaces
of the glass in a state of high compression and the central core in
compensating tension. Heat-treated glass has two compression layers
or zones, one starting at each surface, plus an interior tension
zone centered in the middle of the glass. Each of the two
compression zones is approximately 20% of the glass thickness. The
middle 60% of the glass thickness is the tension zone.

The color, clarity,
chemical composition and light transmission characteristics of glass
remain unchanged after heat-treating. Likewise, hardness, specific
gravity, expansion coefficient, softening point, thermal
conductivity, solar transmittance and stiffness remain unchanged.
The only physical properties that change are improved flexural and
tensile strength and improved resistance to thermal stresses and
thermal shock. Under uniform loading, heat-treated glass is stronger
than annealed glass of the same size and thickness. Heat-treating
glass does not reduce the deflection of the product for any given
load.

Heat-Treated Glass is
separated into two products, heat-strengthened glass
and fully tempered glass, by definition of the degree
of residual surface compression or edge compression. Most furnaces
can produce both. A furnace and its quench must be adjusted by its
operator for one or the other of a product “run.” The adjustments
may include changes in furnace temperature, exit temperature of the
glass, residual time in the furnace, and volume and pressure of the
quench air.

Production of
Heat-Treated Glass
There are two basic methods for producing air-quenched heat-treated
glass. The most commonly used heat-treating furnace, a horizontal
roller hearth, transports glass on horizontal rollers through the
heating and quench processes. A limited amount of heat-treated glass
is produced in vertical furnaces, which call for the glass to be
held in a vertical position by tongs as it is transported through
the heating and quench processes.

Each method produces
some degree of bow and warp, which is an inherent characteristic of
all heat-treated glass. Tong-held glass, the vertical process, may
exhibit a long arc or “S” curve plus some minor distortion at the
tong points. Horizontally heat-treated glass will have
characteristic waves or corrugations caused by the transport
rollers. Industry fabrication requirements, product tolerances and
testing procedures for heat-treated glass are defined in the ASTM
International (ASTM) document C 1048 Standard Specification for
Heat-Treated Flat Glass - Kind HS, Kind FT Coated and Uncoated Glass.

Heat-Strengthened GlassHeat-strengthened glass is produced with surface and edge
compression levels less than fully tempered glass, as specified by
ASTM C 1048. The lower compression levels yield a product that is
generally twice as strong as annealed glass of the same thickness,
size and type. The size and shape of the break pattern of
heat-strengthened glass varies with the level of surface and edge
compression achieved in the heat-treating process. Heat-strengthened
glass with low compression levels will tend to fracture into large
fragments, similar to annealed glass breakage. As the compression
levels increase, the size of the particles of broken glass tend to
become smaller.

ASTM C 1048 requires
that heat-strengthened glass have a surface compression level
between 3,500 pounds per square inch (psi) to 7,500 psi. The break
pattern of heat-strengthened glass is relatively large. The glass
pieces typically remain engaged in the glazing pocket, decreasing
the probability of fall out. Broken glass should be removed and the
opening boarded up or reglazed as soon as possible.

Heat-strengthened
glass does not meet the safety glazing requirements of the American
National Standards Institute (ANSI) Z97.1 American National
Standard for Safety Glazing Materials Used in Buildings - Safety
Performance Specifications Method of Test or the federal safety
standard Consumer Products Safety Commission 16 CFR 1201 Safety
Standard for Architectural Glazing Materials.

Fully Tempered
GlassFully tempered glass is required in ASTM C 1048 to have
either a minimum surface compression of 10,000 psi (69 MPa or an
edge compression of not less than 9,700 psi (67 MPa) or meet ANSI Z
97.1 or CPSC 16 CFR 1201. The higher compression levels yield a
product that is generally four times stronger than annealed glass
and twice as strong as heat-strengthened glass of the same
thickness, size and type.

When broken by
impact, fully tempered glass immediately disintegrates into
relatively small pieces thereby greatly reducing the likelihood of
serious cutting or piercing injuries in comparison with ordinary
annealed glass. To qualify as a safety glazing material as defined
by ANSI Z97.1 and CPSC 16 CFR 1201, the ten largest particles taken
from a broken fully tempered lite of glass shall weigh no more than
the equivalent weight of 10 square inches (64 sq. cm) of the
original specimen when tested according to the standards. Fully
tempered glass that meets ASTM C 1048 does not automatically qualify
as a safety glazing material.

The above information is from the
GANA Glazing Manual, 2004 Edition - the most frequently
referenced resource in the architectural glass and glazing industry.
The Glazing Manual is an excellent addition to any technical
library.
Go here
to order a copy of the manual or CD-ROM. For further information on
this and other GANA reference documents visit the
PUBLICATIONS section of the GANA website.

Chemically Strengthened
Glass
Chemical strengthening of glass is produced
through a process known as ion-exchange. One of the methods used to
chemically strengthen glass calls for the lites to be submersed in a
molten salt bath at temperatures below the strain point of the
glass. In the case of soda-lime float or soda-lime sheet glass, the
salt bath consists of potassium nitrate. During the submersion
cycle, the larger alkali potassium ions exchange places with the
smaller alkali sodium ions in the surface of the glass. The larger
alkali potassium ions “wedge” their way into the voids in the
surface created by the vacating smaller sodium ions.

Chemically strengthened glass
production requirements and test procedures are defined in ASTM C
1422 Standard Specification for Chemically Strengthened Flat
Glass. The specification covers the requirements for chemically
strengthened glass products, which originate from flat glass for use
in building construction, transportation and other specialty
applications.

Under the specification,
chemically strengthened glass is classified on the basis of
independent levels of surface compression and case depth. Increasing
levels of surface compression permit an increasing amount of
flexure. Greater case depths provide increased protection from
strength reduction caused by abuse and abrasion. Consumers should
consult with chemically strengthened glass fabricators regarding the
recommended surface compression and case depth levels required for
their individual application. Product classification levels may be
confirmed through laboratory testing in accordance with the
specification.

Chemically strengthened glass can
be significantly stronger than annealed glass, depending upon the
glass product, strengthening process, level of abrasion, and the
application. Chemically strengthening glass is often the alternative
to thermal tempering when applications call for glass that is very
thin, small in size, or complex in shape.

Although chemically strengthened
glass can be cut after treatment, it is not recommended, as edge
strength will be reduced to that of annealed glass.

When broken by impact, chemically
strengthened glass exhibits a break pattern similar to annealed
glass, and therefore, does not meet safety-glazing requirements in a
monolithic form. When safety-glazing performance is required,
chemically strengthened glass should be laminated.

While chemically strengthened
glass is often used monolithically, product usage has increased in
laminated constructions for security, detention, hurricane/cyclic
wind-resistant, blast and ballistic-resistant glazing applications.

The above information is from the
GANA Glazing Manual, 2004 Edition - the most frequently
referenced resource in the architectural glass and glazing industry.
The Glazing Manual is an excellent addition to any technical
library.
Go here
to order a copy of the manual or CD-ROM. For further information on
this and other GANA reference documents visit the
PUBLICATIONS section of the GANA website.